The occurrence of fireside deposits on heat transfer surfaces in industrial and utility boilers is a persistent problem. The deposits often cause serious loss of heat transfer efficiency, increased corrosion of superheater and boiler tube metals, high operating costs for deposit removal, and plugging of flue gas passages.
These problems are particularly severe in the kraft pulping chemical recovery boiler because of the high ash content (about 35 to 45%) of the fuel, the black liquor comprising spent pulping chemicals from the pulping operation, possibly combined with some bleach plant effluent, and the highly volatile nature of the ash. It has been estimated that about 10 to 20% of the total ash introduced with the black liquor ends up as either carryover particles or fume dust engrained in flue gases. As a result, massive accumulation of deposits from the flue gas on the heat transfer surfaces is not an uncommon occurrence in kraft recovery units, often leading to a complete blockage of the boiler, causing significant production losses associated with unscheduled shutdowns.
Kraft recovery unit deposits consist mainly of sodium sulphate, sodium carbonate, sodium chloride, with a small amount of sodium hydroxide, potassium salts and reduced sulphur compounds. The deposits are formed by two distinctly different mechanisms, namely impaction of carryover particles on heat transfer surfaces (carryover) and deposition by condensed vapours of compounds volatilized in the lower part of the unit (condensation). In the lower superheater region, particularly on the windward side of the tubes, the carryover mechanism is dominant, forming hard and thick deposits. In the upper superheater region, generating section and economizer, deposits are formed mainly by condensation and, under normal conditions tend to be white, friable, powdery and relatively easy to remove.
To prevent the adverse effects of massive deposit accumulation noted above, deposit control is critical to the efficiency and availability of the recovery unit. Deposit accumulation conventionally is controlled in two ways, namely removal of deposits by sootblowers and optimization of the firing conditions in the lower furnace to prevent massive deposit build-up.
To achieve removal of deposit accumulation, sootblowers inject high pressure steam through small rotating nozzles to dislodge deposits from heat transfer surfaces. Sootblowers are operated on various cleaning energy level and blowing frequencies, depending on the location, boiler operating conditions and nature of deposits. Draft loss across the superheater, boiler bank and economizer, and/or flue gas temperatures at the boiler bank and economizer outlets are used as guidance for sootblower operation. A higher blowing frequency normally is required in the generating and economizer sections than in the superheater region, since the flue gas passages in the generating and economizer sections are much narrower and more susceptible to blockage than in the superheater region and, in the superheater region, the gas temperature is high and the deposit melts and ceases growing after building up to a certain thickness.
Acoustic devices or sonic sootblowers, employing low frequency sonic and high power waves, also have been used to remove powdery deposits and dust in the economizer and areas where dry dust prevails. Such devices also may be employed in locations, such as connecting ducts, choppers and precipitators, where steam lances would not be appropriate.
Both steam and sonic sootblowers are generally quite effective in the removal of friable and powdery condensation deposits but are not effective against the hard and tenacous carryover deposits, particularly when there are molten phases involved.
In units experiencing serious plugging problems, the control of massive deposit accumulation by additives in addition to sootblowers has been sometimes attempted. Such additives are believed to modify deposit chemistry, decrease deposit stickiness and tenacity, and improve the deposit removal efficiency of sootblowers. The results of the use of such additives, however, have not been conclusive.
The major difficulty encountered in the deposit control strategy is the absence of effective means for deposit monitoring. The control of deposit accumulations has largely been based on the experience of the individual operators, with the crude information provided indirectly by the measurement of pressure drop or draft loss across the superheater, boiler bank and economizer. When the pressure drop becomes abnormally high, it often is too late to take any preventative action, since most of the flue gas passages will already be blocked and the deposits will have become resistant to sootblowing.
In most recovery units, the deposit accumulation is crudely followed by the operator by monitoring the change in flue gas temperatures at the boiler bank and economizer outlets. At a given black liquor firing rate, higher flue gas temperatures imply more deposit accumulation since less heat has been transferred from flue gas to steam. The flue gas temperature, however, is also significantly influenced by many other operating factors and hence may not be relied upon entirely to indicate the degree of deposit accumulation in the unit.
Further, since plugging and superheater corrosion usually occurs in the superheater and generating sections, the continuous measurement of the flue gas temperature in the superheater region and boiler bank inlet is important and critical to the deposit control strategy. However, as a result of the highly corrosive and dirty environment in these regions, no means of continuous flue gas temperature measurement is presently available.
More recently, computer control systems have been developed to optimize sootblowing and boiler operation. Deposit accumulation is monitored by draft loss, gas temperature drop or heat transfer into the water in the economizer or into the steam in the superheater. However, all these measurements give only crude indications of deposit accumulation, particularly in the case of large boilers.
Optical devices, such as dust sensors, opacity meters and smoke meters, have been used to monitor and control dust and particulate emission. These devices, however, can only to be used at locations after the electrostatic precipitator where the duct is narrow and both dust concentrations and flue gas temperature are low.
As noted above, the prevention or control of deposit accumulation by manipulating boiler operating conditions is universally practised, based on the operator's own experience. Massive deposit accumulation would appear to be caused by a number of variables related to boiler operation, boiler design and deposit control and removal. The variables often interact with one another, with the result that a change in one operating variable can easily affect the others in both constructive and destructive ways, making it difficult to identify the cause of massive deposit buildup.
Resulting from the lack of effective deposit-monitoring devices and scientific guidelines, the prevention of massive deposit accumulation by optimizing firing conditions in the lower furnace has been carried out on a "trial-and-error" basis and has not achieved much success, particularly for units which are overloaded.
Utility and industrial boilers, including coal and oil-fired boilers, and municipal and industrial waste incinerators, also experience problems associated with fireside deposits, particularly decreases in heat transfer efficiency and high temperature corrosion. Plugging problems in these boilers is not the major concern it is in kraft recovery units, because of the much lower ash content of the fuels. The deposits formed in such boilers are usually heavier, hardier and melt at much higher temperatures than kraft recovery unit deposits. In contrast with kraft recovery unit deposits which consist mainly of water-soluble sodium salts, deposits in coal-fired boilers are insoluble, consisting of high proportions of silica, alumina, iron oxides, calcium oxides and sulphate with only a small amount of water-soluble alkali salts. Deposits in oil-fired boilers are similar but also can contain relatively high concentrations of vanadium compounds.
The control of deposits in utility boilers is carried out in much the same way as in kraft recovery units by using sootblowers to dislodge deposits and draft loss and/or flue gas temperature determinations for deposit monitoring. As in kraft recovery units, there is presently no effective means of monitoring deposit accumulation in utility and industrial boilers.
In U.S. Pat. No. 4,408,568 to Wynnyckyj et al, there is described a furnace wall deposit monitoring system using two radiant type heat flux probes, one clean and one fouled by deposits. Although this system can be operated as an on-line instrument to monitor deposit accumulation on the furnace wall, the system cannot be employed to monitor carryover deposits since the heat flux probes are mounted on the furnace wall which is parallel to the flow direction of the flue gas.
As may be seen from the above discussion of the state of the art, there is no direct means of measuring deposit accumulation, so that an operator is not aware of how much carryover there is in the upper part of his boiler at a particular time. This information is particularly important in the kraft recovery unit, since short term variations in the boiler operation can have a dramatic effect on boiler plugging and episodes of high carryover and/or high temperature can quickly plug a boiler.
Accordingly, there is a need for advance deposit control, particularly for kraft mill recovery units, to lower sootblower steam requirements, decrease forced shutdown for recovery unit washouts, improve recovery unit thermal efficiency and increase recovery unit capacity and thereby pulp production capacity.